Development of hybrid cars and electric vehicles has progressed rapidly, which entails the rapid development of electrical brakes.
An electrical application of a brake in a vehicle generally uses a battery as a power source. In such a case, if the battery fails to supply power due to some reason, the brake cannot be applied. Therefore, an auxiliary battery independent of the main battery is included in the vehicle as an auxiliary power supply.
Japanese Patent Application Non-Examined Publication No. H05-116571 discloses prior art to the present invention.
In such a power supply device for vehicles, since the auxiliary power supply is responsible for braking the vehicle when the main battery fails, it is critical to supply power to the auxiliary power supply without fail at an emergency. Therefore, it is important to predict the service life of the auxiliary power supply accurately and keep detecting the status of the auxiliary power supply.
However, if a battery is used as the auxiliary power supply, it is difficult to predict the service life of this battery. While voltage of the battery can be monitored, it is difficult to detect battery abnormality by only monitoring voltage. Thus the battery has been replaced periodically with a new one ahead of time so that the safety can be more positively ensured. The periodical replacement of the battery ahead of time is somewhat effective; however, it is difficult to monitor the status of the battery everyday or every time when a user starts the vehicle. Further improvement is thus demanded.